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From: xclerc <xavier.clerc@inria.fr>
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To: Ashish Agarwal <agarwal1975@gmail.com>
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Subject: Re: [Caml-list] printing an exception changes printed backtrace


Le 14 janv. 2011 à 22:15, Ashish Agarwal a écrit :

> I observe the following unexpected behavior:
> 
> --- a.ml ---
> try
>   … some code ...
> with e ->
> (*  Printf.eprintf "%s\n" (Printexc.to_string e); *)
>   Printexc.print_backtrace Legacy.stderr
> ------
> 
> Compiling as ocamlopt -g a.ml gives:
> $ ./a.out
> Raised at file "fool.ml", line 162, characters 13-22
> Called from file "bar.ml", line 13, characters 5-56
> Called from file "bar.ml", line 43, characters 4-63
> 
> Now this doesn't say which exception was raised, so I uncomment the commented line. I'm expecting the identical output, but with the exception printed first. However, I get:
> 
> $ ./a.out
> Not_found
> Raised at file "hashtbl.ml", line 93, characters 19-28
> 
> So it appears that printing an exception changes the backtrace. Is that possible?
> 
> Unfortunately, I cannot reproduce this behavior in a minimal example. The following script behaves as expected:

It seems possible to reproduce the problem with the following source code:


let custom_printer = true

let () =
  if custom_printer then
    Printexc.register_printer
      (function
        | Not_found -> failwith "silently"
        | _ -> None)

let rec f () =
  ignore (raise Not_found);
  f ()

let () =
  try
    f ()
  with e ->
    Printf.eprintf "%s\n%!" (Printexc.to_string e);
    Printexc.print_backtrace stderr



You will get different backtraces according to the value of "custom_printer".
The underlying problem is that "Printexc.print_backtrace" prints the backtrace
for the last exception raised... which can be one raised by "Printf.eprintf" or
"Printexc.to_string", but locally caught and not exposed to the outside world.

In the code sample above, an exception is raised by the register custom
printer. By design of "Printexc.to_string", such exceptions are silently
ignored... but change the value of the last raised exception (and thus its
associated backtrace).

Your experience of the backtrace actually vanishing may be an instance of
the same problem, where it just happens that the code for the custom printer
has not been compiled with the "-g" option.


Hope this helps,

Xavier Clerc